Chromium-containing metal and manufacturing method thereof

ABSTRACT

The high-Cr-containing metal according to the present invention is a chromium-containing metal manufactured in an arc melting furnace, and contains at least 85% Cr, up to 0.005% Al, up to 0.1% Si, and up to 0.002% S. The manufacturing method of this high-Cr-containing metal comprises the steps of reducing chromium oxides heated and melted in the arc melting furnace with Si, obtaining a molten metal containing at least 85% Cr, then, discharging slag generated in this Si reduction from the arc melting furnace, adding a basic flux into the arc melting furnace after discharging slag, melting the basic flux by electric arc, refining the molten metal by contacting the slag generated through melting of the basic flux with the molten metal, and then, tapping the molten metal from the arc melting furnace and cast.

TECHNICAL FIELD

The present invention relates to high-purity metal chromium andferrochromium used in electronic materials and corrosion-resistant andheat-resistant superalloys and so on, and a manufacturing methodthereof.

BACKGROUND ART

High-purity metal chromium and ferrochromium low in impurities are usedfor electronic materials and corrosion-resistant and heat-resistantsuper alloys. In the present invention, these metal chromium andferrochromium are generically called “chromium-containing metals”. Aneconomically available ore serving as a chromium source of thechromium-containing metal is chromite (FeO.Cr₂O₃). Because it containsmuch iron, however, the upper limit of the Cr content in ferrochromiumobtained from chromite is about 72 mass %. It is therefore the generalpractice to use chromium oxide (Cr2O3) available by refining chromite asa raw material for metal chromium.

The known manufacturing methods of metal chromium include thealuminum-thermit process of reducing powdery chromium oxide with powderymetal aluminum, as disclosed in Japanese Unexamined Patent ApplicationPublication No. 60-36632, the silicon reduction process of meltingchromium oxide in an arc furnace and reducing the same with metalsilicon, as disclosed as a manufacturing method of high-puritychromium-iron alloy in Japanese Examined Patent Publication No. 58-7700,and the electrolytic reduction process of electrolytically reducing achromate solution and causing precipitation of metal chromium on thecathode in Japanese Unexamined Patent Application Publication No.62-47436. These known manufacturing methods have respective advantagesand disadvantages.

In the aluminum-thermit process, for example, it is possible tomanufacture metal chromium easily in a simple equipment. This is howeverbased on a batch process, resulting in a small throughput per batch, alow production efficiency, and the process uses expensive metal aluminumas a reducing agent, thus leading to a high manufacturing cost. Aluminumserving as a reducing agent remains in the manufactured metal chromium.Furthermore, the strong reducing atmosphere causes reduction ofrefractory constituents used for furnace lining, and the reducedconstituents are entangled into the metal chromium, posing problems inpurity.

In the silica reduction process, metal silicon serving as a reducingagent is lower in price than metal aluminum, and can reduce oxygen witha stoichiometrically smaller amount than aluminum. It is thereforepossible to manufacture metal chromium at a lower cost than in thealuminum-thermit process even by taking into account the consumption forheating chromium oxide. In addition, continuous production in an arcfurnace is also possible at a high production efficiency. The siliconreduction process is more advantageous than the aluminum-thermit processalso in this term. It has however a serious quality problem in thatsilicon serving as a reducing agent remains in an amount of about 0.7mass % in the manufactured metal chromium.

In the aluminum-thermit process and the silicon reduction process, ifmetal chromium is manufactured in a state of insufficient reduction byusing a small amount of metal aluminum or metal silicon serving as areducing agent, the amount of aluminum or silicon remaining in metalchromium decreases. However, this leads to a deteriorated reductionyield of chromium oxide and an increase in the amount of oxygen inmanufactured metal chromium, thus causing another problem inmanufacturing cost and in quality.

The electrolytic reduction process permits manufacture of relativelyhigh-quality metal chromium. Because of the use of Cr₂(SO₄)₃ as anelectrolyte, however, manufactured metal chromium contains sulfur asmuch as 0.02 to 0.03 mass %. Further because this is aqueous solutionelectrolysis, the manufactured metal chromium contains from 0.3 to 1mass % oxygen, and from 0.02 to 0.05 mass % nitrogen. Necessity of manytreatments for refining the chromate solution results in economicproblems such as complicated manufacturing steps, a high equipment cost,and a large power consumption.

A ferrochromium having a chromium content of at least 72 mass % cannotbe manufactured in a single run alone of reduction-refining of chromiteas described above. It is therefore the common practice to deiron theonce manufactured ferrochromium through an acid treatment or the like,as is disclosed in Japanese Unexamined Patent Application PublicationNo. 6-4897. However, the conventional deironing treatment, includingthis method, cannot be considered to be high in production efficiencybecause of complicated treatment process.

Requirements for a chromium-containing metal as typically represented bymetal chromium used in electronic materials and corrosion-resistant andheat-resistant superalloys are achievement of a higher purity andreduction of manufacturing cost. Regarding these requirements, thechromium-containing metals manufactured by the conventional methodsmentioned above contain much impurity elements, and requires a highermanufacturing cost according as the purity is higher, and cannot beconsidered to satisfy these requirements.

DISCLOSURE OF INVENTION

The present invention was developed in view of these circumstances asdescribed above, and has an object to provide a method for economicallyand efficiently manufacturing a chromium-containing metal having slightcontents of impurities such as aluminum, silicon and sulfur, and toprovide a chromium-containing metal having small contents of impurityelements, manufactured by this manufacturing method.

The present inventor carried out studies to find a method forefficiently manufacturing a chromium-containing metal having smallcontents of impurities and applicable for electronic materials andcorrosion-resistant and heat-resistant superalloys at a low cost. Theypaid attention to the silicon reduction process having the smallest costvariation from among the conventional arts, and examined a method forremoving silicon remaining in molten metal in the silicon reductionprocess. As a result, the following findings were obtained; silicon canbe easily removed by producing a molten metal by the silicon reductionprocess, and after discharging slag, conducting refining in the presenceof an appropriate flux. The findings suggested the possibility tomanufacture a chromium-containing metal having small contents ofaluminum, silicon and sulfur.

The present invention was developed on the basis of the above-mentionedfindings, and provides a manufacturing method of a chromium-containingmetal, comprising a first step of reducing a chromium oxide with siliconto obtain a molten metal; and a second step of, after discharging slagproduced in the first step, adding a basic flux to refine the moltenmetal.

An embodiment of the manufacturing method of a chromium-containing metalof the invention comprises the steps of reducing a chromium oxide heatedand melted in an arc melting furnace with silicon to obtain a moltenmetal containing at least 85 mass % chromium: then, discharging slagproduced by silicon reduction from the arc melting furnace; newlyadding, after discharge of slag, a basic flux into the arc meltingfurnace to melt the basic flux by electric arc; refining the moltenmetal by contacting slag produced by melting of the basic flux with themolten metal; and then, tapping the molten metal from the arc meltingfurnace and casting the same.

Because CaO has a high desulfurization ability and is available at a lowcost, the basic flux should preferably mainly comprise CaO. Applicationof a heating treatment in a vacuum atmosphere eliminates gasconstituents such as nitrogen and oxygen in the chromium-containingmetal and permits manufacture of a high purity chromium-containingmetal. Therefore, it is desirable to crush the cast chromium-containingmetal, and subject the crushed chromiumcontaining metal to a vacuumheating treatment. By this treatment, it is possible to achieve anitrogen content of up to 0.005% in the chromium-containing metal.Furthermore, it is desirable to crush a chromium-containing metal, formthe crushed metal into briquettes, and subject the resulting briquettesto a vacuum heating treatment.

In the present invention, a chromium-containing metal is manufactured byreducing a chromium oxide such as chromium oxide (Cr₂O₃) or chromite ina first step and refining a molten metal after discharging slag with abasic flux in a second step. Since manufacture is accomplished withoutusing aluminum as a reducing agent, the aluminum content in themanufactured chromium-containing metal can be kept at up to 0.005 mass %(hereinafter simply denoted as “%”). Because silicon is used as areducing agent, while silicon is contained in an amount of about 0.2 to1.0% in the molten metal immediately after reduction-smelting, refiningwith the basic flux finally permits reduction to below 0.1%. Theproduced molten metal is refined with the basic flux. Sulfur containedin the molten metal is therefore removed into the basic flux, and themanufactured chromium-containing metal can have a sulfur content of upto 0.002%.

More specifically, the chromium-containing metal of the inventionsolves, in terms of quality, the problem of the aluminum content in thealuminum-thermit process, the problem of the silicon content in thesilicon reduction process, and the problem of the sulfur content in theelectrolytic reduction process, and in terms of the manufacturing cost,permits continuous production in an arc melting furnace, and productionat a lower cost as compared with the aluminum-thermit process and theelectrolytic reduction process because of the use of low-cost silicon asa reducing agent.

The silicon reduction process conventionally used for the manufacture offerrochromium is a single-stage refining process comprising the steps ofcharging a chromium oxide, silicon and a basic flux into an arc meltingfurnace, and refining the charged raw materials while heating. Themanufacturing method of the present invention, in contrast, is atwo-stage refining method comprising the steps of reducing a chromiumoxide with silicon in the first step, and refining the molten metal witha basic flux after discharging slag in the second step.

The conventional silicon reduction process has a problem of a highsilicon content. If the chromium-containing metal is manufactured in astate of insufficient reduction by reducing the amount of metal siliconas a reducing agent (known as a weak reduction state), there would be adecrease in reduction yield of chromium oxide, although the amount ofsilicon remaining in the metal chromium is reduced to some extent. Asmaller amount of blended metal silicon serving as a reducing agentkeeps a high concentration of chromium oxide in slag during operation.This results in an increase in viscosity of slag during operation, whichin turn leads to a lower rate of the reducing reaction, and hence to ahigher concentration of chromium oxide in slag after operation. Sincethere is a correlation between the amount of chromium oxide in slag andthe oxygen content in metal chromium, there is encountered a problem inthat a smaller amount of metal silicon as a reducing agent results in anincrease in oxygen content in metal chromium.

In the conventionally used silicon reduction process, if a basic flux ischarged in a large quantity at a time for reducing the silicon content,a problem is caused in that the amount of slag increases, and thesilicon content is not stable.

In the two-stage refining method of the present invention, in contrast,produced slag is discharged after the first step. There is a correlationbetween the silicon oxide content in slag and the oxygen content inmetal chromium, and between the chromium oxide content in slag and theoxygen content in metal chromium. Discharge of slag after the first stepcan prevent the oxygen content in metal chromium from increasing. Byrefining the molten metal by use of a basic flux newly charged, it ispossible to reduce not only the silicon content in metal chromium, butalso the oxygen content. The reduction yield of chromium oxide neverdecreases, and the oxygen content in metal chromium is also reduced.

The present invention is particularly suitably applicable to ahigh-chromium containing metal containing at least 85% chromium. Forchromium-containing metals used in electronic materials andcorrosion-resistant and heat-resistant superalloys, there is only asmall demand for ferrochromium of a low chromium grade of under 85%.

The chromium-containing metal of the invention, manufactured in an arcmelting furnace, comprises at least 85 mass % chromium, up to 0.005 mass% aluminum, up to 0.1 mass % silicon, and up to 0.002 mass % sulfur.

An embodiment of the chromium-containing metal of the invention,manufactured in a combination of an arc melting furnace and a vacuumtreatment equipment, comprises at least 85 mass % chromium, up to 0.005mass % aluminum, up to 0.1 mass % silicon, up to 0.002 mass % sulfur,and up to 0.005 mass % nitrogen.

The chromium-containing metal should preferably be manufactured throughreduction of a chromium oxide with silicon.

The manufacturing method of a chromium-containing metal of the inventioncomprises the steps of using, as a raw material, a chromium-containingmetal containing at least 85 mass % chromium, at least 0.005 mass %aluminum, or at least 0.1 mass % silicon, or at least 0.002 mass %sulfur, adding a basic flux to this raw material, and melting andrefining the same in a melting furnace.

Furthermore, the chromium-containing metal of the invention ismanufactured by using, as a raw material, a chromium-containing metalcomprising at least 85 mass % chromium, at least 0.005 mass % aluminum,or at least 0.1 mass % silicon, or at least 0.002 mass % sulfur, addinga basic flux to this raw material, and melting and refining the same ina melting furnace, comprising at least 85 mass % chromium, up to 0.005mass % aluminum, up to 0.1 mass % silicon, and up to 0.002 mass %sulfur.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail. In the invention,chromium oxide (Cr₂O₃) and/or chromite are used as a chromium oxideserving as a chromium source, and at least any one of metal silicon,ferrosilicon, and silico-chromium (Si—Cr) is used as a reducing agent.Since, among these raw materials, chromite, ferrosilicon, andsilico-chromium contain much iron, chromium oxide and metal silicon areused when manufacturing metal chromium. As required, a raw metalmanufactured by the aluminum-thermit process or the silicon reductionprocess may be mixed with chromium oxide as a melting raw material, orfurthermore, any of various metals containing chromium may be mixedwithin a range not deviating from the scope of the present invention.Metal chromium as herein used means a chromium-containing metal having achromium content of at least 99.0%.

When manufacturing a chromium-containing metal having a chromium contentwithin a range of from 85 to 99.0%, i.e., ferrochromium, chromium oxideand metal silicon are used as main raw materials. Because iron may beadded in response to the grade of chromium, the chromium content isadjusted so as to achieve the target content by mixing chromite, ormixing ferrosilicon and silico-chromium. Since chromium oxide has amelting point of 1,990° C., it is difficult to melt it in an arc meltingfurnace unless a flux is added. The reduction efficiency of chromiumoxide is higher according as the basicity of slag (CaO/SiO₂) is higher.In order to ensure operation by melting silicon oxide (SiO₂) producedthrough silicon reduction, a flux should preferably be added. For thesereasons, upon melting chromium oxide, it is desirable to use a fluxmainly comprising CaO such as quicklime or limestone (hereinafterreferred to as a “CaO-based flux”). Use of a CaO-based flux isadvantageous also in that the produced molten metal is desulfurized. Theamount of added CaO-based flux should preferably be such that the slagbasicity upon the completion of silicon reduction is within a range offrom 1.3 to 3.0. A strong reducing agent such as aluminum may be used asa part of the reducing agent to an extent that it does not remain in themolten metal. More specifically, while metal silicon is used as a mainraw material in terms of the kind of reducing agent, aluminum mainlycontaining silicon, or carbon mainly containing silicon may of course beused in mixture in response to the grade and the quantity of heatgeneration. For the purpose of adjusting the slag melting point,fluorspar (CaF₂) or the like may be added in mixture.

Chromium oxide, a reducing agent, and CaO-based flux selected inresponse to the chromium grade are charged in an arc melting furnace,and melted by electric heating. The arc melting furnace should betiltable for tapping the produced molten metal and discharging moltenslag. The arc melting furnace should preferably be of the cartridge typein which the furnace lining is replaceable, for melting while holdingthe raw materials therein so that different products of various gradesof chromium can be manufactured in the same arc melting furnace withoutcausing mutual contamination of constituents. The power source forgenerating arc may be DC or AC, and magnesia stamps or the like may beused as lining refractories.

In the arc melting furnace, contact of a graphite electrode with thecharge, particularly molten metal resulting from the reducing reactionupon heating and melting causes an increase in the carbon content in theresultant molten metal. It is therefore desirable to prevent carbonpickup from the graphite electrode as far as possible by adopting thehigh-voltage operation and using a long distance between the graphiteelectrode and the charged raw materials.

When charging these raw materials into the arc melting furnace, chromiumoxide, the reducing agent, and the CaO-based flux may be charged aftermixing them uniformly. In this case, however, the produced molten metaland the graphite electrode often come into contact with each other.Therefore, for the purpose of preventing carbon pickup from the graphiteelectrode, it is desirable to avoid contact between the produced moltenmetal and the graphite electrode as far as possible by spreading thereducing agent over the hearth, installing the graphite electrode sothat the reducing agent and the graphite electrode are electricallyconnected, charging the mixture of chromium oxide and the CaO-based fluxso as to cover the reducing agent and the graphite electrode, so thatthe reducing reaction occurs below the graphite electrode.

The reducing agent comprising metal silicon, ferrosilicon orsilico-chromium melted by energizing and heating reacts with chromiumoxides comprising chromium oxide or chromite. Reduction by siliconproceeds while generating SiO₂, and a molten metal containing at least85% chromium is produced in the arc melting furnace. Chromium oxidesreact with the CaO-based flux through electrical heating, leading to alower melting point of chromium oxides, thus accelerating the reducingreaction of chromium oxides by silicon. SiO₂ produced from the reducingreaction, the CaO-based flux and the chromium oxides react to formmolten slag (known also as “primary slag”) which cover the molten metal.

In the silicon reduction process, silicon contained in the molten metaland chromium oxide in the molten slag are in equilibrium. That is,addition of the reducing agent in excess for increasing the siliconcontent in the molten metal result in a decrease in the chromium oxidecontent in molten slag. If the silicon content in the molten metal isincreased excessively, however, removal of silicon from the moltenmetal, the next step, becomes complicated. According to experience ofthe present inventors, with a silicon content in the molten metal of atleast 0.2%, chromium oxide in molten slag decreases sufficiently. Evenwith a silicon content in the molten metal of 1.0% or higher, thereducing reaction is saturated, and chromium oxide in molten slag doesnot decrease so much. It is therefore recommendable to achieve a siliconcontent in molten metal within a range of from 0.2 to 1.0% after thecompletion of the reducing reaction by silicon, i.e., to determineblending ratios of metal silicon, ferrosilicon or silico-chromium in thereducing agent so that the silicon content after the completion ofsilicon reducing reaction is within a range of from 0.2 to 1.0%, orpreferably, from 0.4 to 0.8%. The reduction yield in this siliconreduction was confirmed to well bear comparison with that in thealuminum-thermit process.

It is not necessary to charge the reducing agent into the arc meltingfurnace simultaneously with the chromium oxides. The chromium oxides maypreviously be melted in the arc melting furnace, and then, the reducingagent may be added into the arc melting furnace for silicon reduction.

The chromium oxides are thus silicon-reduced, thereby completing thesilicon reduction of the chromium oxides. The timing of completion ofreducing reaction may be determined from chromium analytical value ofmolten metal or molten slag by taking an analytical sample from moltenmetal or molten slag (primary slag), or may be empirically determinedfrom the power turn-on time or the power consumption.

Then, molten slag is discharged by tilting the arc melting furnace. Itis not necessary to completely discharge molten slag generated fromsilicon reduction, but it suffices to discharge more than 50% of primaryslag produced. However, a large quantity of primary slag remaining inthe arc melting furnace leads to a large quantity of basic flux to beadded next. The largest-possible quantity of primary slag shouldtherefore preferably be discharged.

Chromium in an oxide form is contained in an amount of several % in theprimary slag. Non-collection thereof leads to a loss in chromium yield.It is therefore desirable to add metal silicon, ferrosilicon orsilico-chromium into the discharged primary slag to reduce chromiumoxides remaining in the primary slag and collect it as silico-chromium.Collected silico-chromium can be used as a reducing agent of chromiumoxides in the arc melting furnace in place of the above-mentionedreducing agent.

The present inventors have confirmed that slag is not weathered with abasicity of up to 1.5. Therefore, when using primary slag after coolingas a subgrade material, the basicity of primary slag should preferablybe within a range of from 1.3 to 1.7 if chromium is collected by addinga reducing agent to primary slag, or within a range of from 1.3 to 1.5if chromium in primary slag is not collected. When chromium iscollected, reduction with silicon, the basicity of 1.7 is finallydecreases to below 1.5.

After discharging primary slag from the arc melting furnace, a basicflux is added into the arc melting furnace. Any flux mainly comprising abasic constituent such as CaO or MgO may be used as the basic flux. Witha view to desulfurizing the molten metal, a flux mainly comprising CaOsuch as CaO—CaF₂ is preferable.

After adding the basic flux, power is turned on again to melt the basicflux to fuse primary slag remaining in the furnace and the basic flux. Ahigh basicity slag (known as “secondary slag”) is formed, and refiningis performed by bringing the secondary slag and the molten metal intocontact. Contact between the secondary slag added with the basic fluxand the molten metal causes a reaction between silicon and oxygen in themolten metal. SiO₂ is produced, and there is accomplished removal ofsilicon in the molten metal, i.e., a desiliconization treatment. Thereaction in the meantime causes a part of chromium in the molten metalto be oxidized and transfer into slag. If oxygen remains in secondaryslag in the form of Al₂O₃ or SiO₂, carbon of the electrode reduces Al₂O₃or SiO₂, and aluminum or silicon enters into the molten metal asimpurities. To prevent this inconvenience, the largest possible amountof primary slag should preferably be discharged.

In the secondary slag produced by the addition of the basic flux, theslag basicity decreases under the effect of SiO₂ generated from thedesiliconization treatment. In this case, the slag basicity after thedesiliconization treatment should preferably be at least 2.0 for thepurpose of desulfurization of the molten metal. It is thereforedesirable to determine an amount of added basic flux so as to keep aslag basicity of 2.0 after the desiliconization treatment. It is notparticularly necessary to limit the upper limit of basicity. Because thedesulfurization ability is saturated even at a basicity of over 10.0, itsuffices to set an upper limit to up to about 10.0.

Upon the completion of the desiliconization treatment of the moltenmetal by secondary slag, power is turned off, and the molten metal andsecondary slag are discharged into a holding vessel such as a ladle.Determination of the completion of the desiliconization treatment can beaccomplished accurately by taking an analytical sample from the moltenmetal to carry out an analysis of silicon in the molten metal.Completion of the reaction is possible also from analysis of secondaryslag. When the quantity of secondary slag is large, only secondary slagis first discharged, and then, the molten metal is tapped. The tappedmetal is cast from the holding vessel into a mold made of cast iron.Metal chromium has a solidification point of 1,890° C., andferrochromium containing at least 85% chromium has also a high meltingpoint. In order to protect the mold, therefore, it is desirable tospread crushed primary or secondary slag collected during manufacture orcrushed magnesia or the like in the mold. When the molten metal can becast directly from the arc melting furnace into the mold, it is notnecessary to tap once into the holding vessel, but the tapped moltenmetal can be cast directly into the mold.

Secondary slag also contains several % chromium in the form of oxides,and secondary slag has a high basicity. They may therefore be weatheredif held as they are. By collecting secondary slag, and charging thecollected secondary slag together with raw materials such as chromiumoxides into the arc melting furnace to conduct silicon reduction, thechromium yield is improved, and the amount of added quicklime can becurtailed. In the course of this refining, a slight amount ofcarbonaceous material such as coke may be added to adjust the balancebetween the carbon content and the oxygen content prior to the vacuumheating treatment described later.

After casting, the cooled ingot is taken out from the mold. Slagadhering to the ingot surface is ground off by shot blast or the like.Subsequently, the ingot is crushed to a size passing through a mesh ofabout 50 mm by means of several kinds of crusher. The thus obtainedchromium-containing metal has an aluminum content of up to 0.005%, asilicon content of up to 0.1%, and a sulfur content of up to 0.002%. Asa high-purity product, however, the oxygen content and the nitrogencontent must be considered to be slightly higher. These gaseousconstituents are therefore removed by a vacuum heating treatment in avacuum treatment equipment.

A vacuum treatment equipment of any type may be used so far as thepressure in a vacuum vessel is up to 133 Pa (1 torr), and a charge inthe vacuum vessel can be heated to a temperature of at least 1,200° C.

Crushed chromium-containing metal is charged into the vacuum vessel, thepressure is reduced, and heating is started in a vacuum state. Thecharge is held under conditions including a pressure of up to 133 Pa anda temperature of at least 1,200° C. for a prescribed period of time. Asa result of this vacuum heating treatment, in the chromium-containingmetal, oxygen reacts with carbon and the resultant CO gas is removed.Nitrogen is removed as N2 gas, and the nitrogen content becomes 0.005%or lower. The pressure, the temperature and the holding time in thevacuum vessel in the vacuum heating treatment, varying with thecombination of these three factors and specifications of the vacuumtreatment equipment, cannot necessarily be limited in a generalizedmanner. According to experience of the present inventors, it wasconfirmed to be possible to manufacture a chromium-containing metalhaving a desired chemical composition under conditions including apressure in the vacuum vessel of up to 7 Pa (0.05 torr), a temperatureof 1,350° C., and a holding time of 50 hours. After the vacuum heatingtreatment, oxidation of the chromium-containing metal is prevented bycooling the metal in the vacuum vessel to a temperature at which airoxidation is not caused. After cooling, as required, the metal iscrushed to a smaller size to prepare a product. For the purpose ofimproving the degassing efficiency caused by the vacuum heatingtreatment, furthermore, and to improve uniformity of degassing, acrushed chromium-containing metal should preferably be used. Asrequired, it is desirable to add a carbonaceous material serving as areducing agent such as carbon powder, add a lump-making agent (cokingadditive), knead and form the same into briquettes, and then to carryout the vacuum heating treatment.

By manufacturing a chromium-containing metal containing at least 85%chromium, it is possible to efficiently and stably manufacture at a lowcost a chromium-containing metal having a low contents of impurityelements such as aluminum, sulfur, silicon, carbon, oxygen and nitrogen,particularly low contents of aluminum, silicon and sulfur, which isunavailable by the conventional aluminum-thermit process, the siliconreduction process or the electrolytic reduction process.

Phosphorus and iron in the manufactured chromium-containing metal(ferrochromium containing much iron has no problem) are brought aboutfrom chromium oxide, metal silicon and quicklime used as raw materials.It is therefore desirable to select raw materials having low contents ofphosphorus and iron.

Another embodiment of the present invention will now be described. Theaforementioned embodiment has covered a manufacturing method of achromium-containing metal comprising a first step of obtaining a moltenmetal through reduction of chromium oxides with silicon in a meltingfurnace, and a second step of, after discharging slag generated in thefirst step from the melting furnace, newly adding a basic flux into themelting furnace to refine the molten metal. However, achromium-containing metal containing at least 85 mass % chromium, notchromium oxide, and having an aluminum content of at least 0.005 mass %,a silicon content of at least 0.1 mass %, and a sulfur content of atleast 0.002 mass % may be used as a raw material, and this raw materialmay be melted and refined in the arc melting furnace by adding a basicflux thereto. That is, a chromium-containing metal, containingimpurities, manufactured by the aluminum-thermit process or the siliconreduction process may by used as a raw material, and refining only ofthe second step may be carried out by adding a CaO-based flux, fluorsparor the like to the raw material. According to this manufacturing method,it is possible to obtain a chromium-containing metal not containing muchimpurities, including up to 0.005 mass % aluminum, up to 0.1 mass %silicon, and up to 0.002 mass % sulfur.

The present invention is not limited to the embodiments mentioned above,but applicable to various other variants. The invention is applicablefor the manufacture of ferrochromium having a chromium content of under85%. A high-frequency melting furnace, a low-frequency melting furnace,a resistance furnace or a plasma melting furnace other than an arcmelting furnace may be used as a melting furnace. In the second step,after refining the molten metal with a basic flux and discharging slag,it is possible to adopt a multi-stage refining process such as thethree-stage refining process comprising an additional third step ofrefining the molten metal after discharging slag with a basic flux,thereby further reducing the silicon content and the oxygen content.

EXAMPLE 1

The following paragraphs describe an example in which metal wasmanufactured in a 4,000 kVA three-phase AC arc melting furnace having amelting furnace body of the cassette type. This arc melting furnace hasa structure in which only the melting furnace body is tiltable.

Lumpy metal silicon in an amount of 1,350 kg screened to 20 mm under wasspread over the hearth of the melting furnace body, and a graphiteelectrode was installed in contact with this metal silicon. A mixtureprepared by previously mixing 5,000 kg powdery chromium oxide and 4,500kg granular quicklime screened to 20 mm under was charged into thefurnace so as to cover the metal silicon and the graphite electrode.Heating was started by turning on the electrode. When the charge levelin the melting furnace went down as a result of heating and melting,melting was continued while additionally charging the mixture ofchromium oxide and quicklime, and silicon reduction was completed inabout 4 hours and 30 minutes. A basicity of 1.55 was set as a target forthe primary slag upon the completion of silicon reduction.

Primary slag was discharged by tilting the melting furnace body.Discharged primary slag had a basicity of 1.55 as planned, and achromium content in slag of 4.5%. Silico-chromium was collected byadding metal silicon to this primary slag. After collection ofsilico-chromium, the primary slag had a basicity of 1.35 and a chromiumcontent of 0.8%. The primary slag was used as a subgrade material andwas never weathered. The silicon content in the molten metal upondischarging the primary slag was confirmed to be 0.40% from the resultof analysis of a sample taken from the molten metal.

After discharging the primary slag, 600 kg quicklime and 300 kgfluorspar were charged into the melting furnace body as a basic flux,and power was turned on again to melt quicklime and fluorspar to form asecondary slag. The molten metal was refined by means of the secondaryslag. Refining was completed upon the lapse of about an hour and 30minutes from the start of turn-on. The melting furnace body was tiltedwhile disconnecting power. The molten metal was tapped, together withthe secondary slag, and cast into a cast iron mold in which the primaryslag was spread. Upon tapping, the molten metal had a temperature of2,000° C., and the secondary slag had at that moment a basicity of 3.5and a chromium content of 4.0%. The chromium yield was 87% in the firststep, and 85% in the second step. When collecting chromium oxides not asyet collected by adding metal silicon to slag, a total yield of 95% wasobtained.

After cooling, slag adhering to the surface was ground off by shotblast. After grinding, the ingot had a weight of about 3,000 kg. Thepower consumption, as derived from this ingot weight, was 3,500 kWh/tfor the silicon reduction step, and 1,500 kWh/t for the refining stepwith the basic flux, leading to a total power consumption of 5,000kWh/t. The chemical analysis values of the ingot are shown in Table 1.As is clear from Table 1, a metal chromium having a silicon content of0.002%, an aluminum content of 0.001%, and a sulfur content of 0.0002%were obtained. TABLE 1 Chemical composition of metal chromium Chemicalcomposition (mass %) Cr Si Fe P Al C O N S Ingot 99.4 0.02 0.23 0.0030.001 0.14 0.20 0.02 0.0002 Prod- 99.6 0.02 0.25 0.003 0.001 0.008 0.0380.002 0.0002 uct

The metal chromium ingot was crushed to a size smaller than 40 mm bythree kinds of crusher, and charged into a vacuum heating furnace usinga graphite heater. Temperature was raised to 1,350° C. while keeping apressure of up to 13 Pa (0.1 torr) in the vacuum vessel, and the vesselwas held at 1,350° C. for 50 hours. During this holding time, thepressure in the vacuum vessel was lower than 7 Pa (0.05 torr).Subsequently, the vacuum vessel was cooled while keeping a pressure ofup to 13 Pa in the vacuum vessel to room temperature, and the vacuumvessel was released to the open air. An analytical sample was taken frommetal chromium and the chemical composition was analyzed. The result ofanalysis is shown also in Table 1.

As is clear from Table 1, carbon, oxygen and nitrogen were removed bythe vacuum heating treatment, and metal chromium having a very highpurity was obtained. Iron contained in metal chromium in this Exampleresulted from metal silicon in the reducing agent. By using metalsilicon having a high purity, it is possible to reduce also the ironcontent.

A metal chromium ingot in an amount of 300 kg was crushed on a rod millinto a size smaller than 0.5 mm, and carbon powder was blended theretoso as to achieve an atomic ratio of 0.9 relative to oxygen in metalchromium. A PVA 10% solution was mixed in an amount of 3% as alump-making agent, the mixture being compressed and formed intobriquettes, and then dried. The resultant briquettes were held at 1,350°C. under 13 Pa (0.1 torr) for 50 hours, and subjected to a vacuumheating treatment. The result of analysis is shown in Table 2. TABLE 2Chemical composition of metal chromium Chemical composition (mass %) CrSi Fe P Al C O N S Ingot 99.6 0.04 0.24 0.003 0.001 0.061 0.112 0.020.0002 Briquettes 99.7 0.04 0.25 0.003 0.001 0.006 0.008 0.001 0.0002product

EXAMPLE 2

A raw metal manufactured by the aluminum-thermit process in an amount of27 kg, 18 kg quicklime serving as a basic flux, and 12 kg fluorspar weremixed and the mixed raw material was charged in an arc melting furnace.The raw metal was refined through melting by turning on an electrode.Refining reduced the contents of aluminum, silicon and sulfur in the rawmetal as shown in Table 3. TABLE 3 Chemical composition of metalchromium Chemical composition (mass %) Si Al S Raw metal 0.4 0.4 0.03Refined metal 0.04 0.002 0.0002

According to the present invention, it is possible to economically andefficiently manufacture a high-purity chromium-containing metal havinglow contents of aluminum, silicon and sulfur, which was unavailable inthe conventional methods including the aluminum-thermit process, thesilicon reduction process and the electrolytic reduction process, thusproviding industrially useful effect.

1-10. (canceled)
 11. A chromium-containing metal manufactured through amethod comprising the steps of: reducing a chromium oxide with siliconby heating and melting, in an arc melting furnace, the chromium oxidewith sufficient silicon to obtain a primary slag and a molten metalcontaining at least 85 mass % chromium and more than 0.2 mass % and lessthan 1.0 mass % silicon; then, discharging the primary slag from the arcmelting furnace; adding, after discharge of the primary slag, a basicflux into the arc melting furnace to melt the basic flux by electric arcto obtain a secondary slag; refining the molten metal containing atleast 85 mass % chromium and silicon by contacting the secondary slagwith said molten metal to form a refined molten metal with a siliconcontent which is reduced to below 0.1 mass %: and then, tapping therefined molten metal from the arc melting furnace and casting therefined molten metal to form cast chromium-containing metal.
 12. Achromium-containing metal according to 11, wherein said basic fluxmainly comprises CaO.
 13. A chromium-containing metal according to claim11, wherein said chromium-containing metal comprising at least 85 mass %chromium, up to 0.005 mass % aluminum, up to 0.1 mass % silicon, and upto 0.002 mass % sulfur.
 14. A chromium-containing metal according to 13,wherein said basic flux mainly comprises CaO.
 15. A chromium-containingmetal, comprising at least 85 mass % chromium, up to 0.005 mass %aluminum, up to 0.1 mass % silicon, up to 0.002 mass % sulfur, and up to0.005 mass % nitrogen, manufactured through a method comprising thesteps of: reducing a chromium oxide with silicon by heating and melting,in an arc melting furnace, the chromium oxide with sufficient silicon toobtain a primary slag and a molten metal containing at least 85 mass %chromium and more than 0.2 mass % and less than 1.0 mass % silicon;then, discharging the primary slag from the arc melting furnace; adding,after discharge of the primary slag, a basic flux into the arc meltingfurnace to melt the basic flux by electric arc to obtain a secondaryslag; refining the molten metal containing at least 85 mass % chromiumand silicon by contacting the secondary slag with said molten metal toform a refined molten metal with a silicon content which is reduced tobelow 0.1 mass %; tapping the refined molten metal from the arc meltingfurnace and casting the refined molten metal to form castchromium-containing metal; and then, the cast chromium-containing metalis crushed, and the crushed chromium-containing metal is subjected to avacuum heating treatment.
 16. A chromium-containing metal according to15, wherein said basic flux mainly comprises CaO.
 17. Achromium-containing metal, comprising at least 85 mass % chromium, up to0.005 mass % aluminum, up to 0.1 mass % silicon, up to 0.002 mass %sulfur, and up to 0.005 mass % nitrogen, manufactured through a methodcomprising the steps of: reducing a chromium oxide with silicon byheating and melting, in an arc melting furnace, the chromium oxide withsufficient silicon to obtain a primary slag and a molten metalcontaining at least 85 mass % chromium and more than 0.2 mass % and lessthan 1.0 mass % silicon; then, discharging the primary slag from the arcmelting furnace; adding, after discharge of the primary slag, a basicflux into the arc melting furnace to melt the basic flux by electric arcto obtain a secondary slag; refining the molten metal containing atleast 85 mass % chromium and silicon by contacting the secondary slagwith said molten metal to form a refined molten metal with a siliconcontent which is reduced to below 0.1 mass %; tapping the refined moltenmetal from the arc melting furnace and casting the refined molten metalto form cast chromium-containing metal; the cast chromium-containingmetal is crushed; the crushed chromium-containing metal is formed intobriquettes; and then, the resultant briquettes are subjected to a vacuumheating treatment.
 18. A chromium-containing metal according to 17,wherein said basic flux mainly comprises CaO.